Type I IFN induction of the CDK-inhibitor P21^WAF1 is accompanied by ordered G1 arrest, differentiation and apoptosis in the daudi bcell line

We have previously shown that in the human Burkitt lymphoma B cell-line Oaudi, type I interférons (IFN) induce the expression of the cyclin-dependent kinase (cdk) inhibitor p21 that is linked to the G1 arrest induced by the IFN. Treatment of Daudi cells with IFNa has been previously shown to induce a terminally differentiated phenotype that includes reduction in surface IgM and the expression of certain plasma cell-restricted surface antigens. Here, we show that the type I IFN induction of p21 and ensuing cell cycle arrest correlates with the differentiation of Daudi cells and is ultimately followed by apoptosis and cell death. In Daudi cells, p21 is induced by 6-8 hr following treatment with IFN. The expression of p21 paralleled the onset of G1 arrest and the reduction of surface IgM expression which was used as a marker of the differentiation response, and the IFN treated cells acquired a typical plasma cell-like morphology consistent with the progression towards a terminally differentiated state. The type II IFN IFN? which does not inhibit the growth of Daudi cells did not induce the expression of p21 nor affect the expression of surface IgM. Consistent with previous studies, the protein levels of c-myc and cyclin D3 were also found to be rapidly reduced within the same time frame, and the phosphorylation of the retinoblastoma protein, pRB, was found to be impaired. Induction of p21, cell cycle arrest and differentiation of Daudi cells by IFN treatment was followed by apoptosis and cell death as determined by flow cytometry using propidium iodide staining and the TUNEL assay, and morphologic criteria, and was accompanied by the induction of the protease activity of the ICE-family member CPP32 a known mediator of the apoptotic response. Again, the type II IFN IFNywas without effect. These data support the conclusion that in Daudi cells induction of p21 and G1 arrest by type I IFN reflects orderly cell cycle arrest, differentiation and apoptosis characteristic of a terminal differentiation response. Supported by NIH grants "CA 69959 & Cft 38587 to H.M.J.

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Author Amy Claudine Hobeika Surgery, Surgical Sciences